The present invention relates to moving blades of a steam turbine and, more particularly, to a novel construction of the moving blades of the steam turbines. Still more particularly, the invention is concerned with a turbine moving blade having an anti-erosion plate.
The current tendency of increased capacity and compacting of steam turbines gives rise to the demand for larger length of moving blades of the final stage in the low pressure part of the steam turbine, which in turn increases the centrifugal force exerted on the moving blades during rotation of the rotor to such an extent as would not be withstood by 12%-Cr steel which is the typical conventional material of the moving blades of steam turbines.
As will be described later with reference to the drawings, the maximum stress caused in the moving blades by the centrifugal force can be reduced almost to a half by using Ti-alloy steel as the material of the moving blades in place of the conventionally used 12% Cr-steel.
It is, therefore, a current measure to use the Ti-alloy steel as the material of the moving blades of the final stage in the low pressure part of the steam turbine. When the Ti-alloy steel is used as the material of the moving blades of the final stage in the low pressure part of the steam turbine, an anti-erosion plate is welded to the leading edge of each of such moving blades, in order to protect these moving blades against erosion which may be caused by the steam drain particles contained in the steam.
Usually, this anti-erosion plate is made of a Co-W alloy steel. Since this alloy steel has a specific weight of 8 to 8.5 which is about twice as large as that of the Ti-alloy steel, stresses of a high level are generated at the base of the anti-erosion plate. In order to diminish these stresses, it has been proposed to provide a protrusion projecting toward the steam inlet side from the leading edge of the moving blade of the steam turbine at the base of the anti-erosion plate to increase the width of the blade thereby to reduce the stresses generated at the base of the anti-erosion plate.
According to this countermeasure, however, it is necessary that the above-mentioned protrusion has a considerably large size to make a blade width sufficiently large to uniformly eliminate a drastic increase of the stresses at the base of the anti-erosion plate. This inconveniently results in a large reduction of the turbine efficiency due to the profile loss. It is also to be pointed out that the erosion is increased at the protrusion formed on the leading edge of the moving blade. In addition, the deformation of the moving blade during welding is increased because of the increase of the length of weld between the anti-erosion plate and the moving blade made of Ti-alloy steel.